Raptor 312 is a Raptor piloted by Lt. Sharon "Boomer" Valerii and ECO "Helo" Agathon. It flies in formation with Capt. Lee Adama and one of Galactica's Viper squadrons in the ship's decomissioning ceremony.
It is later called into duty with Capt. Jackson Spencer's ill-fated squadron during the first hours of the Cylon attack. The ship is pursued by Cylon Raiders and damaged by shrapnel from a missile, forcing it to land on Caprica. Its crew makes repairs and saves a select few civilians by lottery, including Gaius Baltar. Helo stays behind on Caprica in exchange for the extra weight of Baltar, believing that Baltar's genius was more important to the fleet than himself.
Raptor 312 is known to a few crew members as having a possible "Gimbal Haunting" where Raptor 314 causes freshly rebuilt gimbals to appear 'bad' even after being tested multiple times both before and after the alleged 'bad gimble' event was reported to have happened - the gimble is always blamed for the skid marks left by the in the landing deck by Raptor 314 with both pilot and ECO reporting trouble with the gimble
Although she has piloted other Raptors (such as when she flew in Raptor 478 for a time after the attacks - perhaps while Raptor 312 was being repaired), Boomer may have been assigned Raptor 312 or have a preference for it as her call sign BOOMER is part of Raptor 312's livery and is clearly visible printed under the canopy
When CPO Galen Tyrol refers to a Raptor which Valerii had piloted on 47 missions, he may be referring to Raptor 312 (TRS: "Flight of the Phoenix"). She is also known to have piloted Raptor 478 ("33," "Water").